In my experiences at the cop shop, there’s a single, glaring tactical mistake that I see nearly every day from both rookies and grizzled old veterans. This mistake is one of those things that is extremely simple in principle, yet overwhelmingly difficult in execution. The best way to describe the problem is to steal a fractured phrase from famed Yankee slugger Yogi Berra, who said “It ain’t over ‘til it’s over.”

In other words, you can never let your guard down even if it appears that everything is wrapped up and the world is a happy place.

This seems like such a basic concept, but in the heat of battle, I’ve seen it ignored too many times to count. It’s even possible that I may have committed the same sin a couple thousand times.

I suppose it’s just human nature to mentally “stand down” when you believe things have stabilized after a critical event. However, those with enough real-world crisis experience know that this is the exact moment to get worried. Handshakes and high-fives can come later.

Regardless if you’re a beat cop, member of the military, security officer, or private citizen, this principle applies directly to you. After the initial phase of any incident has passed, the first thought that should always cross your mind is that somewhere “out there” is another problem waiting for the right moment to cancel all your lifetime subscriptions.

That problem could be a potential assailant who was missed during an initial search, reinforcements for the original bad guy, or another non-human danger such as a secondary explosive device or unseen physical hazards.

In a non-tactical example, think of the countless people who have survived bad automobile accidents without a scratch only to inadvertently walk into oncoming traffic. Just because you survived the first threat doesn’t mean that the world is suddenly full of roses and lollipops.

Hopefully both regular readers of this column understand the point. The problem is training our minds to think in these terms.

Fortunately, there’s a fairly simple way to do this: make sure you have the “1 + 1” rule burned into your memory, posted on your car’s dashboard, and tattooed on the inside of your eyelids.

For those who aren’t familiar with it, the “1+1” rule states that no matter how many bad guys, weapons or other problems you locate, there is always one more. For example, if you find a weapon on a suspect, you had better assume there is another one hidden nearby.

Recently I read a report of a shooter who had three handguns concealed on his body, along with a couple of knives. Such things are uncommon, but not rare. If the officers involved had simply said, “Ah-ha! I found the bad guy’s gun” and stopped searching after the first, second, third, or fourth weapons, someone could have had a nasty surprise later.

With apologies to the psychologists out there trying to help us, adhering to the 1+1 rule requires the cultivation of a serious mindset of paranoia and distrust. When you hear things like, “We’ve got all the suspects in custody,” or “This AO has been cleared of all hostiles,” you must instinctively categorize such statements as merely a belief, not fact.

Obviously, we should only deal with facts when discussing dangers to our own tender epidermis.

The 1+1 rule very much applies to the bad guys. There are innumerable instances in my own career where a suspect unexpectedly appeared as we were patting down his partner. Equal in number are those times when a criminal materialized while the written report was being taken 20 minutes after an incident.

A few years ago in this corner, I wrote about my first-ever capture of a burglary suspect. After sweeping a bar where we had found an unlocked door following an alarm, my supervisor reminded me to check the restrooms. It would be interesting if we could travel back in time and see who screamed more loudly and highly pitched when the door was flung open: the equally surprised and thoroughly drunken intruder or I.

I was lucky in that instance because I had completely let down my guard after we had already checked the building once. Had the suspect been intent on killing a cop, it is unlikely these words would have ever been written. Fortunately, he just needed a drink.

Never, ever assume that a bad guy has fled the scene—because they often don’t. Whether you’re a dumb rookie cop checking a tavern alarm or a homeowner who pulls into his driveway to find his house burglarized, don’t ever fall into the trap of thinking that the suspect is miles away. They might only be feet—or inches—away.

If you’re a cop, you’ve probably seen that suspicious persons are notorious for appearing unexpectedly after a call is concluded. It’s common that, once officers conduct an initial sweep of the area, someone will stumble on the suspicious person half an hour later in the exact same location.

Sometimes the number of suspects can surprise you. There was one incident on the interstate several years ago when I pulled 22 people from a single van. That was stretching our concept somewhat thin, but it still applied.

It doesn’t matter if you’re talking about the number of people, weapons or other hazards: letting your guard down after an incident is a natural, but sometimes fatal, human tendency. Resolve to never allow yourself the luxury of thinking that everything is warm and fuzzy while you’re still on the potential killing ground.

Otherwise, another famous Berra quote could be applicable: “Always go to other people’s funerals, otherwise they won’t come to yours.”